1. Field of the Invention
The present invention relates to a conveyance apparatus configured to convey a sheet in form of a roll.
2. Description of the Related Art
In an image recording apparatus (printer) equipped with a conveyance apparatus, images and characters are recorded on a continuous sheet paid out from a roll sheet. In this printer, the conveyance and stop of the sheet by a conveyance roller are repeated for the purpose of recording, and the recording is performed with ink discharged from a recording head configured to perform scanning in a direction crossing the conveyance direction while the sheet is stopped.
In such a apparatus, a metal shaft passes through the rotation center of a roll member, around which the continuous sheet is wound, and the roll member is rotatably set in the printer via the metal shaft. At the core of this metal shaft, there is provided a torque limiter for applying a braking force. As the roll sheet is conveyed, a constant back tension is applied to the roll sheet (See Japanese Patent Application Laid-Open No. 2005-096987).
In the printer, when the roll sheet is paid out by a conveyance roller by a length needed for recording, the roll member may continue to rotate for some time due to the inertial weight of the roll member around which the roll sheet is wound. In such cases, the roll sheet is paid out by an excessive amount from the roll member, resulting in generation of slack in the roll sheet. The slack amount varies depending on the inertial weight of the roll member and the peripheral speed of the conveyance roller. The larger the inertial weight of the roll member, and the higher the peripheral speed of the roll member, the greater the slack generated in the roll sheet paid out from the roll member.
In the case where the roll sheet is greatly slackened immediately after the conveyance operation, no tension is generated in the roll sheet at the time of the next conveyance operation. Then, as the roll sheet is conveyed, the slack is gradually reduced. At the moment that the slack is eliminated, there is generated large tension in the direction opposite to the conveyance direction of the roll sheet. During this conveyance, slack is generated in the roll sheet again.
On the other hand, in the case where the slack immediately after the conveyance operation starts is small, the slack is eliminated during the next conveyance operation, and large tension is generated in the direction opposite to the conveyance direction of the roll sheet. Therefore, the slack in the roll sheet varies depending on the inertial weight of the roll member and the peripheral speed. Each time the conveyance operation is performed, the tension acting on the roll sheet changes.
This change in the tension acting on the roll sheet greatly affects the conveyance precision of the roll sheet. In the case where the tension of the roll sheet is large, the slippage of the roll sheet on the conveyance roller increases during conveyance operation, and the conveyance amount of the roll sheet decreases by an amount corresponding to this slippage. Even when the tension is large, when the tension is constant, it is possible to prevent variation in the actual feeding amount by adding a predetermined fixed correction value to the feeding amount. However, in the case where slack is generated in the roll sheet in the conveyance operation, the tension changes for each conveyance operation, so that it is rather difficult to correct the feeding amount by the predetermined correction value, resulting in deterioration in feeding accuracy.
To address this problem, a printer is proposed which rewinds back the roll sheet prior to the start of the conveyance operation based on an estimated length of slack in the roll sheet which can be generated after the conveyance of the sheet by the conveyance roller by an amount corresponding to the estimated slack length (See Japanese Patent Application Laid-Open No. 2008-155417).
By eliminating the slack in the roll sheet prior to the start of the conveyance operation, it is possible to reduce the change in the tension generated during the conveyance operation, so that it is possible to secure the requisite feeding accuracy by correcting the feeding amount by the predetermined correction value.
However, in the configuration discussed in Japanese Patent Application Laid-Open No. 2008-155417, when winding-back the roll sheet in order to remove slack from the roll sheet, a fixed winding-back force is applied. Thus, in the case of a slippery roll sheet, the roll sheet slips on the conveyance roller after the slack has been removed. As a result, the nip position of the roll sheet held between the conveyance roller and the pinch roller can be deviated. In this case, a non-image area appears at a joint portion between recorded images for each reciprocating operation of the recording head, generating white stripes in the recorded images. Further, in the case of a sheet of high stiffness, the slack cannot be sufficiently removed, and, at the time of the next conveyance operation, reverse tension due to the slack is produced, so that in some cases, black stripes are generated at the joint portion between the images due to deficiency of conveyance.